Summary: Little is known about adolescents' electronic cigarette (EC) use and whether EC are a gateway to tobacco or additional nicotine use. Responding to the immediate need for better epidemiological data on all tobacco/nicotine products, the National Institute on Drug Abuse and Food and Drug Administration initiated the Population Assessment of Tobacco and Health study (PATH), a prospective study with a U.S. national sample of 13,651 adolescents (12-17 years) and a parent/guardian of each adolescent. We will use of four waves of annual restricted PATH data with each adolescent and parent/guardian interview 12 months apart. These data will allow us to examine: (1) EC, CC and dual EC/CC use, and consequent DSM-5 Tobacco Use Disorder (TUD); (2) High risk subsamples of adolescents and the risk and protective factors associated with EC use as well as the health consequences associated with EC use; and (3) Parent/guardian data that focuses on their child's health and psychosocial behaviors. Unlike other national, longitudinal studies that include EC variables, we will be able to conduct analyses that include high risk subgroups of 12 to 17 year olds based on sexual minority status, race/ethnicity, sex and geography as well as the inclusion of parent/guardian data regarding health. This study will provide timely information about EC use among adolescents, including never studied subsamples (e.g., sexual minorities). AIM 1: Examine the patterns of EC, CC and dual EC/CC use associated with lifetime, annual and 30-day use, cessation, DSM-5 tobacco use disorder (TUD) symptom severity and the co-occurrence of EC, CC and EC/CC use with other substances (e.g., other tobacco/nicotine products, alcohol, marijuana, etc.). AIM 2: Identify between-person differences among adolescents based on cross-sectional and longitudinal patterns of EC, CC, dual EC/CC and other tobacco/nicotine products (e.g., hookah, bidis, etc.). AIM 3: Characterize within-person lifetime and annual changes in EC, CC, dual EC/CC and other nicotine use related use over time. We will also test for the effect of longitudinal changes in EC, CC, and other tobacco/nicotine use on health consequences. AIM 4: This is a theory-testing aim. We will conduct theory and data source triangulation to increase confidence in the validity of our conclusions by minimizing the inadequacies of a single-source of data, or a single theory. We will use data source triangulation to address the hypotheses proposed by Problem Behavior Theory using PATH data and Monitoring the Future (MTF) data (a school-based sample from 8th and 10th graders). We will conduct a theory triangulation by applying Problem Behavior Theory and Theory of Planned Behavior to the interpretation of PATH data. This project will be the first theory-based study to use four waves of PATH data to prospectively identify risk and protective systems, patterns of tobacco/nicotine use, and health/life compromising outcomes related to adolescents' EC use. The study will provide critical new knowledge and will address the research priorities advanced by the public health community and the National Institutes of Health.